U.S. Pat. No. 3,833,408, herein incorporated by reference, describes the application of methyl alkyl siloxane compositions of the formula ##STR2## wherein R is an alkyl group of 4-20 carbon atoms and x is an integer, as lubricants for conductive video discs comprising a molded plastic disc having audio and video information in the form of geometric variations in a spiral groove in the surface of the disc. These discs are coated first with a conductive material which acts as a first electrode, then with a dielectric layer and a final layer of lubricant. A metal tipped stylus acts as a second electrode of a capacitor and the information signals are monitored by the stylus which notes changes in capacitance between the stylus and the disc surface as the information, in the form of depressions, passes beneath the stylus.
Further developments in this system have produced a video disc which is made of a conductive plastic material, e.g., a PVC copolymer resin containing sufficient amounts of conductive carbon particles so that the disc can provide capacitance readout, while the plastic resin surrounds the carbon particles providing a dielectric surface layer on the conductive particles. This development has eliminated the need for separate coatings of metal and dielectric on the plastic disc.
Video discs are also being developed which do not require a conductive surface or a grooved surface, the stylus being maintained in synchronization with the information pattern by means of electrical signals rather than the groove walls.
These changes in the materials used for the video disc have somewhat changed the requirements for the lubricant and in certain respects the commercially available methyl alkyl siloxane lubricant, sold by the General Electric Company as SF-1147, wherein R in the above formula is a decyl radical and x is about 2-7, is now unsatisfactory.
This material contains an antioxidant compound which is added as ##STR3## A portion of this antioxidant reacts with residual hydride groups in the methyl alkyl siloxane lubricant, to form chemically bound compounds of the type ##STR4## The presence of the unbound, free antioxidant compound can cause changes in the video disc surface with time, contributes to inhomogeneities in the disc surface from one lot to another, particularly in the wetting and lubrication dynamics during playback of the disc, and thus its presence is undesirable. However, in removing the unbound antioxidant, the basic structure of the methyl alkyl siloxane lubricant should remain the same, particularly with respect to molecular weight and molecular weight distribution of the methyl alkyl siloxane molecules. Further, a suitable method for removing antioxidant must not further contaminate the siloxane lubricant with difficult to remove solvents or other material. Further, it is desired that the chemically bound antioxidant also be unaffected to prevent future oxidation or degradation of the lubricant. The present method purifies the methyl alkyl siloxane lubricant, removes chemically unbound antioxidant but without adversely affecting the lubricant or the chemically bound antioxidant so as to improve the performance of this class of lubricants for the video disc application.